Chronic obstructive pulmonary disease (COPD) is emerging as an important cause of morbidity in HIV-infected patients, likely due to multiple factors, including an increased prevalence of smoking, chronic inflammation and immune activation, oxidant stress and respiratory infections. Our preliminary data suggest that several lung matrix metalloproteinases (MMPs) are upregulated in HIV-infected smokers, which could contribute to accelerated emphysema by virtue of their ability to degrade extracellular matrix and basement membrane components. Based on these observations, we hypothesize that pharmacologic inhibition of MMPs by doxycycline, the only FDA-approved inhibitor of MMPs, will favorably modify the natural history of COPD in HIV-infected patients. To test this hypothesis, we propose conducting a phase II randomized, placebo-controlled trial of doxycycline for COPD in HIV-infected patients. Our research team, led by a pulmonologist / translational researcher with expertise in HIV-associated COPD and an infectious diseases specialist/clinical trials expert, is well positioned to propose such a trial to the NIH-funded AIDS Clinical Trials Group (ACTG). To achieve this end, we first propose to conduct a pilot study to enable us to address the following Specific aims: Aim 1. To determine the safety, tolerability, and biologic effects of twice daily doxycycline for 6 months in HIV-infected subjects with COPD; and Aim 2. To prepare and submit an application for a phase II clinical trial of doxycycline for COPD in HIV-infected subjects. To address the first aim, we will conduct a randomized, double-blind, placebo-controlled pilot study of doxycycline 100 mg twice daily in 30 HIV-infected subjects with COPD (2:1 doxy:placebo). The primary endpoint will be safety/tolerability and secondary endpoints will include change in FEV1, reduction of MMP activity in epithelial lining fluid and cells obtained by bronchoscopy and doxycycline levels in blood, ELF and BAL cell pellets. In addition to providing novel insights into the biologic effects of doxycycline in the lung, the pilot study will inform selection of endpoints for a phase II trial, which ultimately will address an unmet medical need for novel interventions for COPD/emphysema in HIV-infected patients.